Powder metallurgy processes and products



United States Patent ice 7 Claims. c1. 75-206) This invention pertains to the art of powder metallurgy, more especially as applied to metals which are highly reactive chemically, such as titanium, zirconium and base alloys of each, and to new materials of superior characteristics formed thereof and processes for producing the same by novel powder metallurgy techniques.

A primary object of the invention is to provide materials of such chemically reactive metals in the manner above referred to which possess superior hot strength properties as compared to those of these same metals as produced in massive form directly from an initially molten state.

In accordance with the basic procedure of the invention this is accomplished by reducing the metal to an extremely fine particle size, ordinarily on the order of 1' to 2 microns, more or less, in mean minimum dimension, applying to each particle a thin, coating of a stable metal oxide, and thereupon consolidating by pressing and sintering.

As is now well known, finely powdered aluminum upon air heating to develop an oxide coating on each particle followed by pressing, sintering and working, results in a material of greatly enhanced strength, creep and fatigue resistance at elevated temperatures as compared to the aluminum metal itself as obtained, for example, by casting from the molten state. The useful service temperature of aluminum is thus raised from ZOO-400 F. to about 600-800 F. Sintered oxidized aluminum powder thus has much higher elevated temperature strength than does aluminum itself or any of its wrought and cast alloys. Worked shapes of the material have some residual ductility-enough to be definitely useful.

The reason for this is not too well understood. One explanation is that by starting with aluminum in fine particle size of the order aforesaid and applying to each particle a thin oxide coating, as by heating in air, and thereupon consolidating by pressing, sintering and working, the oxide coating in each particle breaks up into small, discrete platelets which distribute themselves like fence pickets about the particles, leaving the intervening portions of the aluminum particles exposed to each other to be Welded together and coalesced by the pressing, sintering and subsequent hot and/ or cold working operations. The oxide platelets, however, prevent recrystallization of the small aluminum particles, thus preventing grain growth from particle-to-particle, thereby retaining the original microstructure. The slip shear strength is thus increased as compared to that of massive aluminum as produced from the molten state. This latter on cooling produces relatively large crystals, which reduce the slip shear strength, for this reason and presumably also for the reason that the recrystallization probably re- 3,181,947 Patented May 4, 1965 moves some voids between crystals thus further reducing the slip shear strength.

It has been attempted to produce analogous results with titanium powder, using controlled oxidation, nitriding or carburizing, to produce a skin or adherent coating on each powder particle. Since, however, extremely fine powders must be used, if at sintering temperatures any diffusion occurs, a homogeneous alloy will result and the purpose defeated. Unfortunately, the oxide, nitride and carbide of titanium are readily soluble in titanium and diffuse rapidly into the metal at minimum sintering temperatures of about 1300-l400 F. Instead of sintering to an adherent mixture with properties governed by the duplex structure, the composite homogenizes at least in part and a normal alloy results. The methods applicable to finely divided aluminum are thus not satisfactory when applied to powdered metals such as titanium, zirconium and base alloys of those metals.

In accordance with the present invention, it is proposed to coat the titanium or zirconium particles with. a metal oxide other than an oxide of the base metal particle, which oxide is insoluble in the metal at sintering temperatures. Furthermore, the metal oxide with which the titanium particles, for example, are coated must be one which is stable in contact with titanium at the elevated temperature. That is to say, the metal oxide must not decompose thus providing oxygen for combination with titanium at the elevated temperature.

The oxides of calcium, magnesium, beryllium and thorium fulfill both stability and insolubility requirements for use with titanium and zirconium and base alloys of these metals, and may be employed individually or in admixture in the present invention. Certain of the rare earth oxides also exhibit the necessary characteristics required of the metal particle coating. Gadolinium, for example, is entirely astisfactory. However, for economic reasons the alkaline earth metal oxides mentioned and thorium oxide will generally be employed.

The particles of the base metal are very finely divided, preferably having a mean minimum dimension of the order of about 12 microns or less. In order to assure the requisite coating of the finely divided metal, the particles of metal oxide must have a mean maximum diameter substantially less than the mean diameter of the metal particles. To produce a substantially continuous coating on the metal particles, the metal oxide particles should be of the order of about ten times smaller than the metal powder. Colloidal suspensions of the coating materials provide an excellent source of the metal oxide in a sufficiently fine state of subdivision.

Coating may be effected by agitating the base metal powder in a colloidal suspension. When coating has been efiected, the suspending liquid is drawn off and the thus coated metal powder dried, after which it is consolidated by pressing and sintering. The so-coated particles are'consolidated into an integral composition by con current pressing at about 5000 to 10,000 p.s.i. at sintering temperatures of about 1300-2000 F., and preferably at about 1400-1700 F. over a period of about 5 to 25 hours. Alternatively, pressing and sintering may be carried out as separate steps, in which case, the coated metal powder isfirst consolidated by cold pressing, re-

3 quiring about 75,000 to 125,000 p.s.i., followed by sintering in the temperature range aforesaid.

Because of the ready solubility of nitrogen and oxygen in the base metal, sintering is effected in an atmosphere not containing appreciable quantities of these elements, in order to obtain maximum uniformity of properties in the sintered product. Preferably, the atmosphere is inert with respect to titanium, for example, as argon. The method of the present invention thus departs from the teachings of the sintered aluminum powder metallurgy art wherein the aluminum particles with a surface coating of aluminum oxide are sintered in an oxidizing atmosphere. The problem of solubility of surface metal oxide coating is not encountered in the aluminum powder metallurgy art.

der, for example, titanium or a titanium base alloy. The

mixture is thereupon dried with accompanying continuous agitation, and the residual salt of the preci itating agent leached out with a non-aqueous solvent. The bound water in the particle coating is then drawn off by heating in a vacuum, and the coated metal powder thus obtained is ready for use in the production of pressed and sintered powder metallurgy products in accordance with the invention.

In accordance with another modification, the metal oxide in an extremely fine state of subdivision in the form of a dry powder may be thoroughly admixed with the base metal powder as by ball milling or tumbling, and the resulting mixture pressed and sintered as aforesaid. This thorough admixing coats the base metal particles with the finely divided metal oxide in a loosely adherent manner, which, however, functions in the same manner as the colloidally applied coating to provide powder metallurgy products of improved high temperature properties on subsequent pressing, sintering and working.

Thus, for example, magnesia or one or more of the other metal oxides above referred to may be ball milled, either dry or in admixture with a liquid such as alcohol, a liquid hydrocarbon, or even cold water, provided steps are taken to rid the hydrate formed of its water which would be strongly bound. With any of these modifications the ball milling is continued until the coating of the finely divided metal oxide is produced upon the metal particles or until the metal oxide is reduced to a state of subdivision fine enough to coat the metal particles more or less continuously. At this juncture, titanium, for example, is added and the ball milling continued until microscopic examination reveals that the base metal particles have acquired a more or less continuous coating of the metal oxide. The coating metal oxide when reduced to a sufl'iciently fine state of subdivision will adhere to the base metal particles sufiiciently well to permit removal of the excess metal oxide by screening or blowing. In the event ball milling is carried out in a liquid medium, the excess slurry is drained off following coating of the base metal particles, and the residual mass dried, whereupon excess coating powder is removed by screening or blowing as aforesaid. The material thus obtained is ready for use in the production of pressed and sintered products. If the liquid medium employed is water, a final drying is effected by mild heating in a vacuum prior to consolidation.

It is difficult to prescribe the ultimate thickness of the metal oxide coating because of the permissible variation of particle size and the starting metal oxide itself. Similarly, it is equally diificult to specify weight ratios of metal to metal oxide powder. However, it is sufiicient to '3- say that the concentration of metal oxide in the finished sintered product should be such that the product does not contain more than about 20% by weight oxygen, and preferably less than 20% by weight oxygen.

This application is a divisional application of my copending application for United States Letters Patent Serial No. 634,156, filed January 15, 1957, now Patent No. 3,066,391 which is a continuation-in-part of my application Serial No. 420,440, filed April 1, 1954, now abandoned.

I claim:

1. The process which comprises: applying to finely divided particles of a metal selected from the group consisting of titanium, zirconium and base alloys of said metals, a surface coating of a metal oxide which is stable in contact with and substantially insoluble in said metal at sintering temperatures, and consolidating the so-coated particles into an integral mass with the application of pressure and heat at sintering temperatures.

2. The process which comprises: applying to finely divided particles of metal selected from the group consisting of titanium, zirconium and base alloys of said metals, a surface coating of metal oxide particles of maximum dimension substantially less than the minimum dimension of said metal particles and selected from the group consisting of the oxides of calcium, magnesium, beryllium, thorium, gadolinium and mixtures thereof, and consolidating the so-coated particles into an integral mass by application of pressure and heat at sintering temperatures.

3. The process which comprises: thoroughly admixing finely divided particles of a metal of the group consisting of titanium, zirconium and base alloys of said metals with particles of an oxide of a metal selected from the group consisting of calcium, magnesium, beryllium, thorium, gadolinium and mixtures thereof more finely divided than said metal particles, whereby said metal particles are individually coated with said metal oxide, separating said coated metal particles from non-adhering metal oxide, and thereupon consolidating the so-coated particles by application of pressure and heat at sintering temperatures.

4. The process which comprises: thoroughly admixing particles of a metal of the group consisting of titanium, zirconium and base alloys of each having a mean dimension of the order of 1-2 microns with particles of an oxide of a metal selected from the group consisting of calcium, magnesium, beryllium, thorium, gadolinium and mixtures thereof, said metal oxide particles having a maximum dimension less than about one-tenth that of said metal particles, whereby said metal particles are individually coated with said metal oxide particles, separating the coated particles from non-adhering metal oxide particles and thereupon applying pressure at sintering temperatures, whereby said so-coated particles are consolidated.

5. The process which comprises: thoroughly admixing finely divided particles of a metal of the group consisting of titanium, zirconium and base alloys of said metals with particles of an oxide of a metal selected from the group consisting of calcium, magnesium, beryllium, thorium,

gadolinium and mixtures thereof more finely divided than said metal particles, whereby said metal particles are individually coated with said metal oxide, separating said coated metal particles from non-adhering metal oxide, and thereupon consolidating the so-coated particles by application of pressure and heat at sintering temperatures in an atmosphere which is inert with respect to said metal.

6. A process for the production of a sintered compact which comprises: thoroughly admixing particles of a metal selected from the group consisting of titanium, zirconium and base alloys of each having a mean dimension not greater than about two microns with particles of a metal oxide characterized by substantial insolubility in said metal and thermal stability in the presence of said metal at the sintering temperature, said oxide particles having a mean maximum dimension substantially smaller than that of said metal particles, whereby said metal particles are individually coated with said oxide, and thereupon consolidating the so-coated particles by application of pressure and heat at the sintering temperature in an atmosphere which is inert with respect to said metal.

7. A process for the production of a sintered compact which comprises: thoroughly admixing particles of a metal selected from the group consisting of titanium, zirconium and base alloys of each having a mean dimension not greater than about two microns with particles of a metal oxide characterized by substantial insolubility in said metal and thermal stability in the presence of said References Cited in the file of this patent UNITED STATES PATENTS 2,431,660 Gaudenzi Nov. 25, 1947 

1. THE PROCESS WHICH COMPRISES: APPLYING TO FINELY DIVIDED PARTICLES OF A METAL SELECTED FROM THE GROUP CONSISTING OF TITANIUM, ZIRCONIUM AND BASE ALLOYS OF SAID METALS, A SURFACE COATING OF A METAL OXIDE WHICH IS STABLE IN CONTACT WITH AND SUBSTANTIALLY INSOLUBLE IN SAID METAL AT SINTERING TEMPERATURES, AND CONSOLIDATING THE SO-COATED PARTICLES INTO AN INTEGRAL MASS WITH THE APPLICATION OF PRESSURE AND HEAT AT SINTERING TEMPERATURES. 